1. Field of the Invention
This invention relates to optical scanning devices used for laser machining. More particularly, it relates to a high precision laser hole drilling and controlled material removal of geometries of less than 500 microns.
2. Description of the Prior Art
Galvanometer scanners have been used for nearly three (3) decades for laser material processing. They are most commonly used for laser marking. They have less utility in fine machining applications, in particular drilling precision holes and features below 250 microns, because their positional accuracy is limited below such threshold. Non-galvo-based approaches are limited to circular features and tend to be more optically and mechanically complex. Galvanometer-based systems are the simplest and least expensive way to direct a focused laser beam over a wide area. Nevertheless, they lack the “localized” precision for finite features over a large field.
Thus there is a need for a galvanometer-based apparatus that includes localized precision for finite features over a large field.
A conventional multi-mirror galvometric system positions a focused laser beam by moving the mirrors by means of vectors. There are no “true arcs” generated for circular features. Instead, a circle is approximated by a series of short vectors. It is very difficult to form precision holes or any arc feature below 200 microns in diameter. Moreover, the angular resolution of the galvo motors is a further hindrance to the problem of small features and the attainment of high repeatability.
Thus there is a need for a system that is not detrimentally affected by the formation of circles and arcs through a series of short vectors.
The known systems are also subject to limited angular resolution and thermal drift which further hinders the ability of the device to machine precision features over a long period of time, e.g., a single production shift in manufacturing.
There is a need, therefore, for a system that is less subject to the effects of limited angular resolution and thermal drift so that features can be machined over relatively long periods of time.
A need therefore exists for a device having an improved angular resolution relative to the known devices that have a pair of galvometrically driven mirrors.
Rotating, offset wedge pairs allow good precision below 250 microns, but they only permit circular features and have a limited dynamic range. The focus lens itself can be placed offset from the optical axis and rotated or even placed in an open frame X-Y stage used to make all conceivable geometries but mounting a lens in such a way is bulky and limited over the area that can be machined due to common lens aberrations.
There is thus a need for a device not subject to the limitations of rotating, offset wedge pairs or a rotating, offset focus lens.
Another known method, disclosed in U.S. Pat. No. 4,079,230, includes a pair of matched optical wedges that are rotationally offset and rotated in unison at high speeds. The offset of the matched wedges causes an angular displacement of the laser beam from the original optical axis. This angular deviation causes a lateral displacement of the focal spot when the angularly displaced beam is passed through a focus lens. The difficulty with this technique is that it is hard to coordinate the two wedges precisely at the high rotational speeds or to rapidly change the desired angle of deviation during such rotation. This technique usually requires a multitude of wedge pairs to cover a wide diameter range. The requirement to change wedge pairs adds significant time to replace and align; it is therefore unsatisfactory for most production processes. This method also limits the geometries to circular patterns only.
Other methods include “wobble plates” disclosed in U.S. Pat. Nos. 4,940,881 and 6,501,045 that provide circular and tapered features only and have limited workability in imaged based optical systems.
Another method, disclosed in U.S. Pat. No. 4,896,944, employs an offset focus lens that is rotated to displace the focused spot radially from the optical axis. Such systems are bulky but have utility when fixed diameter holes are required. They lack utility in creating complex features or tapers.
Thus there is a need for a system that has increased versatility relative to the known systems. More specifically, there is a need for a system that can provide complex shapes, including tapers and other non-circular shapes, and which can form features less than two hundred fifty microns (250 μm).
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the identified needs could be met.